1. Field of the Invention
The invention concerns an arrangement for detecting winding shorts in the rotor winding of electrical machines by analyzing shaft voltages or shaft currents.
The invention relates to the state of the art as given, for example, in the paper "Rotor winding short detection" in "IEE Proceedings", Vol. 133, Pt. B, No. 3, May 1986, pp. 181-189.
2. Discussion of Background
A winding short in the rotor winding of an electrical machine causes an asymmetrical current distribution between the generator poles. As a consequence of this winding short, the following effects occur
asymmetric heating of the rotor PA0 forces due to the radial magnetic asymmetry PA0 winding currents in the stator parallel conductors PA0 magnetization of the shaft due to axial magnetic asymmetry. PA0 Detecting the winding short by measuring the asymmetry of the stray magnetic field by means of a measurement coil installed in the air gap of the machine (near the rotor surface). The stray magnetic field is directly proportional to the ampere-turns in a groove. PA0 Detecting the winding short during operation by measuring the winding currents in the stator parallel conductors. If there is rotor magnetic asymmetry due to the winding short, different voltage components are induced in the stator parallel conductors. PA0 Detecting the winding short by subjecting the rotor winding to voltage pulses and analyzing the variation with time of the impulse waves.
The importance of detecting rotor winding shorts in good time, and of repairing the winding short, increases with the size of the machine.
Experience has shown that winding shorts can lead to serious faults if the operational constellation is critical.
Three main measurement methods can be used to identify winding shorts during operation and to observe their change with time:
These and other measurement methods are described in the paper "Rotor winding short detection" in IEE Proceedings, Vol. 133, Pt. B, No. 3, May 1986, pp. 181-189.
The method using a measurement probe in the air gap involves intrinsic dangers, demands an expensive design solution and requires relatively complicated analysis of the measurement signals. The second method also requires a design solution, for the application of the two high-tension-insulated measurement coils (Rogowski coils) wound around the stator conductors, and harmonic analysis of the stator current.
This method is questionable for machines already in operation.